1. Field of the Invention
The invention relates to a controller for a motor drive control system, and more particularly to a control of an alternating current (AC) motor that uses a rectangular-wave control and a pulse width modulation (PWM) control.
2. Description of the Related Art
A drive method using an inverter is generally employed for drive controlling an AC motor by using a direct current (DC) source. The inverter is switching controlled by an inverter drive circuit, and a voltage switched, for example, according to the PWM control is applied to the AC motor.
In order to increase a voltage utilization gain and obtain a high output in a region with a high motor revolution speed, an overmodulation PWM control and a rectangular-wave control are used for an AC motor control by a modulation system in which a fundamental wave component of a motor application voltage is larger than that in the sine-wave PWM control.
Japanese Patent Application Publication No. 2006-230079 (JP-A-2006-230079) discloses an inverter control system in which the disappearance of a pulse component from a PWM signal in the zero cross vicinity where a polarity of a voltage command in the overmodulation mode changes in an inverter PWM control that drive controls an AC motor is inhibited, thereby making it possible to change smoothly the motor output between a sine-wave mode (PWM control) and a one-pulse mode (rectangular-wave control).
In JP-A-2006-230079, in the overmodulation mode, a three-phase voltage command is converted into a step-wave voltage having a high-level value (duty ratio 100%), a low level (duty ratio 0%), and a middle level (duty ratio 50%). A zero cross point can be correctly ensured by setting the zero cross point of each phase voltage in the time central position of the middle level period. Therefore, it is possible to inhibit the pulse disappearance from a PWM signal in the zero cross vicinity and reduce a positive-negative imbalance of each phase voltage.
Further, Japanese Patent Application Publication No. 11-146501 (JP-A-11-146501) discloses a controller such that when switching from a PWM pulse mode to a one-pulse mode (rectangular-wave control) is conducted in an inverter PWM control that drive controls an AC motor, the excitation current component is maintained constant at all times and a torque current component at a maximum torque output is made larger than the excitation current component by a value equal to or greater than a predetermined value till the output voltage of a power converter reaches a maximum voltage that can be outputted by the converter, and when the output voltage reaches the maximum voltage in response to the speed of the motor, the PWM pulse mode is switched to the one-pulse mode.
With the controller described in JP-A-11-146501, when switching is conducted from the PWM pulse mode to the one-pulse mode, a smooth torque control and continuous operation of power converter (inverter) can be realized, without causing a voltage discontinuity.
In the AC motor control that uses the PWM control and rectangular-wave control, when switching is conducted from the rectangular-wave control that controls a voltage phase to the PWM control based on current feedback, a control is sometimes conducted by which the control mode is actually switched in a point slightly shifted from the original snitching point in order to prevent the so-called chattering, that is, frequent switching between the two control modes.
In such a control, the current command in the PWM control changes discontinuously immediately after switching to the PWM control. As a result, the output voltage of the inverter that is applied to the AC motor can vary in a stepwise manner. Such a discontinuous variation of current command is followed by excessive current feedback control that can result in overcurrent, overvoltage, and torque surge. As a result, a stress applied to the inverter or AC motor is increased causing deterioration of the device and the torque can fluctuate instantaneously. In a case where the motor drive control system is carried on an electric vehicle, such torque fluctuations can create a sense of discomfort in the vehicle occupants.